1. Field of the Invention
The invention concerns an antenna arrangement for a magnetic resonance apparatus.
2. Description of the Prior Art
Magnetic resonance apparatuses for the examination of patients, in particular by magnetic resonance tomography, are generally known, for example from DE10314215B4.
Modern magnetic resonance systems (also called MR or MRT systems) normally operate with multiple different antennas (also called coils in the following) to emit radio-frequency pulses for nuclear magnetic resonance excitation and/or to receive induced magnetic resonance signals. A magnetic resonance system frequently has a larger coil, known as a whole-body coil (also called a body coil), that is normally permanently installed in the apparatus, as well as multiple small local coils (also called surface coils). In contrast to the whole-body coil, the local coils serve to acquire detailed images of body parts or, respectively, organs of a patient that are located relatively close to the body surface. For this purpose, the local coils are applied directly at the point of the patient at which the region to be examined is located.
Antennas having what is known as a “transmission line” shape (for example in an open or more closed stripline style) with application in MR imaging are generally known. A very particular type of this antenna shape is taken by what is known as the aperiodic antenna, in which the structure is not resonant but rather represents a transmission line that is terminated correctly in terms of impedance at its end. The impedance termination (termination with an impedance resistor) leads to the situation that no standing wave forms along the antenna. The reflection-free, terminated conductor with its characteristic impedance, which conductor serves as an antenna structure, is traversed by an advancing wave, and the field that is relevant for the imaging is generated evanescently (as a field decaying with increasing distance from the conductor) by the current flowing in the antenna structure without any resonance step-up in the immediate proximity of the conductor.
In the reception mode, the antenna functions reciprocally relative to the transmission phase in that the reflection-free terminated conductor receives MR relaxation signals arising in its surroundings without resonance step-up and transfers them for further processing to the preamplifier connected with it with correct (matched) impedance.
In the following an element of an MR antenna is generally discussed. An MR antenna arrangement typically consists of an entire series of such antenna elements that are arranged around the volumes to be examined. The type and the design of the arrangement can be different. Via a suitable chaining of such antenna elements, either planar structures (planar coils) or those that surround the volume to be examined (volume coils) can be built. In the transmission case, all antenna elements are thereby fed separately with correct phase, and in the reception case the antenna elements are connected by separate preamplifiers with the receiver of the MR system under consideration of the phase relationships.
The advantages of this antenna shape are low costs, simplicity, and due to the spatially limited antenna characteristic, adjacent elements couple only very weakly with one another, and thus the chaining of multiple such antenna elements in 50 ohm engineering is very simple (array formation). Another advantage is broadband capability (with regard to the spectroscopy of interest). Moreover, due to the broadband capability, all calibration and tuning tasks that otherwise would be necessary for a resonant antenna are not needed.
A disadvantage of this antenna form is lower efficiency, because the largest portion of the RF power generated by the amplifier is dissipated (released) thermally by the termination resistors in the transmission case and does not contribute to the generation of the B1 field that is necessary for the imaging. An additional cause of the low efficiency is the limited reflux [return] space that is necessary to achieve the comparably low characteristic impedance of 50 ohms of the transmission line.
Another disadvantage is limited antenna characteristic.
The B1 field generated by such an antenna in the transmission phase is limited to the immediate area in proximity to the antenna structure (the evanescent field of the transmission line that directly penetrates the conductor structure in the adjacent medium). In the reception mode, the signal yield is limited to a superficial region in proximity to the antenna structure due to the same, limited antenna characteristic.
The antenna form has previously always been used with a transmission line that is adapted to the common 50 ohm impedance of the RF portion of the MR system. A very simple design is thereby achieved because the antenna (that now has 50 ohm ports) can be connected directly to the system. Complicated modifications are thus omitted. (See ISMRM Abstract #435 of 2008.) However, the use of a 50 ohm transmission line as an antenna element also entails disadvantages.